Polyoxymethylene resins have been conventionally known as engineering resins having balanced mechanical properties and excellent moldability and have been used, in various fields, for parts of automobiles, parts of electric and electrical devices, industrial goods, toys and the like.
Of these, small-sized and mass-produced articles are mainly produced by an injection molding method. On the other hand, large-sized or non-mass-produced articles are produced by an extrusion molding method and the resultant articles are usually subjected to a secondary process such as cutting. The main articles produced by the extrusion molding method include an article in the shape of a round bar having a diameter of 10 to 200 mm (hereinafter a rod stock), an article in the shape of a sheet having a thickness of 10 to 100 mm (hereinafter a thick sheet) and the like.
When molded articles such as a rod stock and a thick sheet are produced with a polyoxymethylene resin by an extrusion molding method, the resultant articles are not satisfactory, that is, the moldability which the polyoxymethylene inherently possesses cannot provide desirable molded articles. Inside the resultant articles, a part whose color tone is different from that of the peripheral part (hereinafter a whitened part) or a void part comprising microvoids and the like occurs. These parts are unfavorable in view of physical properties and appearance of the molded articles. For example, in case of a screw prepared from a rod stock by cutting, its mechanical strength such as flexural strength, tensile strength and impact strength are reduced at a void part when the void part is large. As a result, the screw is easy to break. In order to solve such problems, improvement in reducing the occurrence of the void part has been awaited.
There has not been any prior art trying to solve the above problems of whitened parts or void parts occurring in a rod stock, etc. Japanese Unexamined Patent Publication No. 33465/1988 discloses a molded article in the shape of a rod stock produced from a resin composition which is prepared by adding ultra high molecular weight polyethylene to a polyoxymethylene resin. However, the purpose of this invention is to improve the resin composition so as not to slip on the screw of the extruder at molding. Therefore, the whitened part and the void part occurring inside the molded article cannot be sufficiently improved.
Further, in order to improve the physical properties of the polyoxymethylene resin, various attempts have been carried out. Of these, several attempts to add a polyolefin and a sterically hindered phenol compound to a polyoxymethylene resin have been proposed. For example, International Patent Publication No. WO92-07033 discloses a composition that comprises a polyoxymethylene polymer and linear low density polyethylene for improving elongation and also describes how sterically hindered phenols may be incorporated. Japanese Unexamined Patent Publication No. 52166/1975 discloses, in the Example, a method for incorporating polyoxymethylenediacetate, fine powder low density polyethylene, 2,2'-methylene-bis-(4-methyl-6-t-butylphenol), a polyamide copolymer and isoeicosanolacetate. Japanese Unexamined Patent Publication No. 103556/1975 discloses, in the Example, a method for incorporating polyoxymethylene diacetate, low density polyethylene, 2,2'-methylene-bis-(4-methyl-6-t-butylphenol), a polyamide copolymer and isostearyl alcohol polyethylene oxide. Japanese Unexamined Patent Publication No. 197452/1984 discloses, in the Example, a method for incorporating a polyacetal ethylene copolymer, 2,2'-methylene-bis-(4-methyl-6-t-butylphenol), nylon 6-6 and an acetal block copolymer.
According to these methods, rub resistance, hard wearing property and impact resistance are improved to a certain degree. However, a whitened part or a void part occurring inside the molded article, which is unfavarable in view of physical properties and appearance of the molded article, is not sufficiently lessened.
The present invention has been conducted under such circumstances to provide a polyoxymethylene resin composition and an extrusion molded article thereof. The extrusion molded article obtained from the polyoxymethylene resin composition of the present invention has smaller whitened and void parts internally which, when not so small, are unfavorable in respect of the physical properties and appearance.